The ability to conduct high-speed data communications between remotely separated data processing systems and associated subsystems and components has become a requirement of a variety of industries and applications such as business, educational, medical, financial and personal computer uses, and it can be expected that current and future applications of such communications will continue to engender more systems and services in this technology.
Associated with such applications has been the growing use and popularity of the “Internet”, which continues to stimulate research and development of advanced data communications systems between remotely located computers, especially communications capable of achieving relatively high-speed data rates over an existing signal transport infrastructure (e.g., legacy copper cable plant).
One technology that has gained particular interest in the telecommunication community is digital subscriber line (DSL) service, which enables a public service telephone network (PSTN) to deliver (over limited distances) relatively high data bandwidth using conventional telephone company copper wiring infrastructure. DSL service has been categorized into several different technologies, based upon expected data transmission rate, the type and length of data transport medium, and schemes for encoding and decoding data.
Regardless of its application, the general architecture of a DSL system essentially corresponds to that diagrammatically shown in FIG. 1, wherein a pair of remotely separated mutually compatible digital communication transceivers are coupled to a communication link, such as a twisted pair of an existing copper plant. One of these transceivers, denoted as a ‘west site’ DSL transceiver 11, is typically located in a digital subscriber line access multiplexer (DSLAM) 12 at a network controller site 13 (such as a telephone company central office (CO)). The other transceiver, denoted as an ‘east site’ DSL modem 21, may be coupled with a computer 22 located at a customer premises 23, such as a home or office.
Within the communication infrastructure of the telephone company, the ‘west site’ DSLAM 12 is coupled with an associated network ‘backbone’ 15, which communicates with various information sources 31 and the Internet 33. This telecommunication fabric thus allows information, such as Internet-sourced data (which is readily accessible via the backbone network 15), to be transmitted from the central office DSL transceiver 11 over the communication link 10 to the compatible DSL transceiver 21 at the customer site 23.
In a DSL system of the type described above, the data rates between DSL transceivers are considerably greater than those for voice modems. For example, while voice modems typically operate at voice frequency band, from DC up to a frequency on the order of 4 KHz (with data rates around 28 Kbps), DSL data transceivers may operate in a bandwidth between 25 KHz to well over 1 Mbps, with data rates typically greater than 200 Kbps and up to 50 Mbps (as in the case of a Very-high-data-rate Digital Subscriber Line (VDSL)). This voice/data bandwidth separation allows high-speed data transmissions to be frequency division multiplexed with a separate voice signal over a common signal transport path.
Moreover, the high-speed frequency band used for ADSL data communications may be ‘asymmetrically’ subdivided or separated (as per (1998) ANSI standard T.413) as shown in FIG. 2, to allocate a larger (and higher frequency) portion of the available spectrum for ‘downstream’ (west-to-east in FIG. 1) data transmissions from the central office site to the customer site, than data transmissions in the ‘upstream’ direction (east-to-west in FIG. 1) from the customer site to the central office.
As a non-limiting example, for the case of a single twisted copper pair, a bandwidth on the order of 25 KHz to 125 KHz may be used for upstream data transmissions, while a considerably wider bandwidth on the order of 130 KHz to 1.2 MHz may be used for downstream data transmissions. This asymmetrical downstream vs. upstream allocation of ADSL data bandwidth is based upon the fact that the amount of data transported from the central office to the customer (such as downloading relatively large blocks of data from the Internet) can be expected to be considerably larger than the amount of information (typically e-mail) that users will be uploading to the Internet.
Fortunately, this relatively wide separation of the upstream and downstream frequency bands facilitates filtering and cancellation of noise effects, such as echoes, by relatively simple bandpass filtering techniques. For example, an upstream echo of a downstream data transmission will be at the higher (downstream) frequency, when received at the central office, so as to enable the echo to be easily filtered from the lower (upstream) frequency signal. Frequency division multiplexing also facilitates filtering of near-end crosstalk (NEXT), in much the same manner as echo cancellation.
In addition to ADSL, there are a number of other DSL technologies, such as High-Bit-Rate Digital Subscriber Line (HDSL), Symmetric Digital Subscriber Line (SDSL), and Very-high-data-rate Digital Subscriber Line (VDSL). Also, HDSL2 (ANSI Standard T.418 (2000)) uses one twisted pair for full duplex 1.544 Mbps payload delivery up to a distance on the order of 18 kft.
Among these, HDSL, unlike ADSL described above, has a symmetric data transfer rate—communicating at the same speed in both upstream and downstream directions. Currently perceived data rates for HDSL are on the order of 1.544 Mbps of bandwidth; however HDSL requires more signal transport infrastructure—two copper twisted pairs. In addition, the operating range of HDSL is more limited than that of ADSL, and is currently considered to be effective at distances of up to approximately 12,000 feet or less, beyond which signal repeaters are required.
SDSL (which is described in ITU standards publications) delivers symmetric data transfer speed that is comparable to HDSL2; however, as pointed out above, it employs only a single twisted copper pair; consequently, its range is currently limited to approximately 10,000 feet. Rates of SDSL are dependent upon line characteristics, such as wire gauge, bridge taps, etc. SDSL may employ rates greater than HDSL2 on short twisted pairs.
VDSL provides asymmetric data transfer rates at considerably higher speeds, e.g., on the order of 13 Mbps to 52 Mbps downstream, and 1.5 Mbps to 2.3 Mbps upstream, which severely limits its range (e.g., 1,000 to 4,500 feet).
In addition to performance considerations and limitations on the transport distance for DSL communications over a conventional twisted-pair infrastructure, the cost of the communication hardware is also a significant factor in the choice of what type of system to deploy. Indeed, a lower data rate DSL implementation may provide high-speed data communications, for example, at downstream data rates on the order of or exceeding 1 Mbps, over an existing twisted-pair network and at a cost that is competitive with conventional non-DSL components, such as V.90, V.34, and ISDN modems (28.8 Kbps to 128 Kbps). ISDN is occasionally referred to as IDSL and is considered by some as a DSL technology.
Still, many telecom service providers currently desire to deliver relatively low cost (repeaterless) ADSL service over extended distances (e.g., on the order of 25 kft). Hence, there is a need for an ADSL line extender.